Method of improving bond strength



March 24, 1970 K. A. SLAYGLE ETA 3,502,148

METHOD OF IMPROVING BOND STRENGTH 7 Filed Jan. 27. 1967 if-ji-Fif 5% Z a .1 I E2- 615 Awe:

(war ,4 iwi ($4149 62 rAziz /f United States Patent O 3,502,148 METHOD OF IMPROVING BOND STRENGTH Knox A. Slagle and Lloyd G. Carter, Duncan, Okla, as-

signors to Halliburton Company, Duncan, kla., a corporation of Delaware Filed Jan. 27, 1967, Ser. No. 612,232 Int. Cl. E21b 33/13, 43/24 US. Cl. 166-292 5 Claims ABSTRACT OF THE DISCLOSURE In cementing a well casing in a formation, the thermal stability of the bond between the casing and the cement which fills the annulus between the casing and the formation wall is improved by first anchoring the lower portion of the casing, e.g., cementing the lower portion of the casing, applying tension to the casing at its upper end and cementing the remainder of the casing in the formation while the casing is held under tension.

BACKGROUND OF THE INVENTION This invention relates to a method for cementing wells and more particularly to a method for forming a strong, thermally stable bond between a well casing or other pipe and the wall of a bore hole.

An oil well may be drilled through permeable formations containing fluids such as fresh water, salt water, and gas. In placing a pipe or casing in a bore hole in such a formation, it is common to place a cement between the casing and the wall of the bore hole to prevent the migration of fluids from the permeable formation into the oilproducing zone. Cementing also seals the producing formation so that it will not lose product into other permeable zones having less pressure. Additionally, cementing is used to bond the pipe or casing to the bore hole.

A typical cementing procedure involves, as a first step, running the pipe to be cemented to the desired depth. Various equipment may be used to guide and center the pipe within the bore hole. Cement is then placed in the annular space between the pipe and the bore hole wall by various techniques, e.g., introducing cement into the pipe thus forcing it down the pipe and then up through the annular space between the pipe and the bore hole wall. After the cement is in place, it is normally allowed to set at the ambient temperature of the formation.

It has recently become common to give cemented wells a high-temperature treatment to lower oil viscosity and thereby increase production rates. Common high-temperature treatments include intermittent or continuous treatment with hot water or steam and in situ combustion. During these treatments, steam pressures of 2500 to 3000 p.s.i. may be used with temperatures of from 500 to 655 degrees F. or higher.

It has been found that the heat treatments frequently result in a failure of the pipe-to-cement or pipe-to-formation bond. Many failures have been found to be due to loss of cement bond or pipe failure at pipe joints. Failure of the cement bond permits fluid communication between the producing zone and zones containing undesirable fluids. Pipe growth may also result with ultimate failure by buckling or telescoping. As a result of the high-temperature treatments, there has been an increased emphasis on forming a strong cement bond.

SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide a method of forming a strong, thermally stable cement bond between a bore hole wall and a pipe.

Briefly, the present invention involves prestressing a pipe in a bore hole during the cementing operation. The process comprises anchoring the lower portion of the pipe in a bore hole, e.g., by cementing the lower portion of the pipe in the hole and then applying tension to the upper portion of the pipe as cement sets around the upper portion of the pipe. Cementing can be accomplished by introducing two types of cement into the annulus between the casing and the formation. A rapid setting cement may be introduced into the casing-formation annulus surrounding the bottom of the casing and a slow setting cement may be introduced adjacent to the upper portion of the pipe. Then, after the rapid setting cement has set to anchor the lower portion of the pipe, tension can be applied while the slow setting cement adjacent to the upper portion of the pipe is still setting.

BRIEF DRESCRIPTION OF THE DRAWING FIGURES 1 to 6 are schematic views illustrating a well in longitudinal cross sections. FIGURES 1 to 3 each illustrate a step in the process of this invention. FIGURES 4 to 6 each illustrate a step in alternative embodiment of the process of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The first step of the invention is to anchor a casing or pipe at its bottom end in a well bore.

This may be conveniently done by cementing the casing at the bottom of the well bore. However, any other convenient method of attachment may also be used.

The method of cementing the lower end of the casing in the well bore can be readily understood by reference to the drawing. FIGURE 1 illustrates a casing 1 within a well formation indicated generally by 2 and having walls 3. When the casing is in place as shown in FIG- URE 1, a slow setting cement 4 may be pumped down the casing and out into the annulus between the casing and the wall of the formation, Immediately afterthe slow-setting cement, a rapid-setting cement 5 is pumped down the casing. If desired, the slow-setting cement may be kept separated from the rapid-setting cement by a plug 6. As the rapid-setting cement rises in the casingformation annulus it will displace the water, drilling fluid or other material 7, which had occupied the annulus.

As indicated in FIGURE 2, the rapid-setting cement will displace the slow-setting cement up into the casingformation annulus and will surround the lower portion of the pipe. The rapid-setting cement is forced down the casing by use of water, drilling mud, or any other suitable fluid. The quantity of rapid-setting cement and slowsetting cement should be such that when the rapid-setting cement has reached the bottom of the casing, it will surround the lower portion of the casing in the casing-foe mation annulus, with the slow-setting cement in contact with the upper portion of the casing for a major portion of its length.

FIGURE 3 illustrates the final step of the process. The rapid-setting cement which has become set 8 anchors the casing in the bottom of the formation. The slow-setting cement has not yet become set. Upon setting of the rapid-setting cement, tension is applied at the top of the casing 9. The means for applying the tension may be jacks, elevators on the rig, or any other suitable means.

An alternative method of adding the cement in the practice of this invention is illustrated by FIGURES 4 to 6. In this method, the casing 1 has a collar 10 with ports 11. In FIGURE 4, the ports are closed as rapidsetting cement 5 is forced down the casing by fluid 7. A diaphragm 12 follows the rapid-setting cement down the casing so that it will become lodged at the stops 13,

' aphragm may follow directly after the rapid-setting cement if desired.

When the diaphragm becomes lodged as indicated in FIGURE 5, the ports 11 in the collar are opened. Then, slow-setting cement is pumped down the casing so that it will flow out of the ports and fill the casing-formation annulus above the rapid-setting cement. Any slow-setting cement remaining in the casing can be flushed out with a fluid such as water. After the slow-setting cement is in place, diaphragm 12 may be broken by applying an increased pressure to the fluid in the casing. After the rapidsetting cement has set as indicated in FIGURE 8, tension is applied to the casing at 9 and maintained while the slow-setting cement is setting.

Any of the well-known techniques of cementing well casings may be applied to the present invention. Thus, a chemical wash may be used ahead of the cement to condition the formation surface and remove drilling mud which would interfere with the bond between the casing and the formation. Additionally, wiper plugs may be used in adding the cement. A wiper plug may be in the general form of a cylinder forced down the casing by fluid pressure. The wiper contacts the walls of the casing and cleans off any mud or other undesirable foreign material. The plugs may have diaphragms set to rupture at a given pressure so that when they have reached the bottom of the casing they can be broken to permit communication between the casing and the casing-formation annulus.

In general, any amount of stress and tension applied to the casing during setting of the slow-setting cement will result in improved bond strength during heat treatments. The preferred amount of stress is that which will result in an elongation of the casing at least equal to the elongation which would occur in an uninhibited pipe in being heated from the normal temperature of the formation to the temperature obtained during heat treatment. This stress and the load required to produce it can be readily calculated using standard engineering calculations and the thermal and mechanical properties of the materials involved.

The amount of rapid-setting cement used should be suflicient to anchor the lower portion of the casing securely in the well formation. Preferably, the rapid-setting cement should contact as small a portion of the casing which is exposed to the heat treatment as possible.

In general, any type of cement which will anchor the casing in the formation may be used for the cement identified in this application as the rapid-setting cement. If the cement which anchors the casing is introduced into the formation by the method illustrated in FIGURE 1 to 3, it should be a cement which sets more rapidly than the cement which is in contact with the remainder of the casing. However, it should be understood that the terms rapid-setting cement and slow-setting cement are relative terms. Thus, the term rapid setting cement is merely used to indicate that the anchoring cement sets rapidly in relationship to the rate of setting of the cement which is in contact with the remainder of the pipe. Thus a cement which actually is considered to set at a moderate speed or even a fairly slow speed can be used to anchor the pipe as long as the other cement sets at a slower rate yet. Furthermore, the cement used to anchor the pipe could set at the same rate or even a slower rate than the cement which is in contact with the upper portion of the pipe during tensioning if the method of introducing cement shown in FIGURES 4 through 6 were employed. Thus, although additional time might be required, one could wait for cement in FIGURE 5 to become completely set before introducing cement 4. In the case, the relative rate of setting of the two cements would not be critical. By the use of accelerators and other means, those skilled in the art can modify the setting time of the cement in order to obtain cements having the desired characteristics for use in the present invention.

A suitable cement which can be used as the rapidsetting cement is a calcium aluminate cement such as Lumnite containing an accelerator. The calcium aluminate cement may also contain about 40% silica flour.

A slower-setting cement which may be used in this invention is an API Class G cement containing about 30% to 50% silica flour. A perlite may also be added at a 1:1, or 2:1 ratio. In addition, additives such as cement friction reducers, bentonite, and chemical accelerators may be used.

The stress is applied to the casing until the cement has completely set.

Any type of metallic casing may be used in this invention. A pipe of API J-55 steel should prove suitable for process.

The following example will further illustrate the invention.

Example I The process of this invention .was used to set a casing in a well located in Santa Barbara County, Calif. A hole 9%" in diameter was drilled to a depth of 2,378 feet. A 7" casing with buttress threads of N-8O steel was then set to a depth of 2,258 feet. The casing had a solid nose guide shoe, three centralizers one of each of the first, third and fifth joints, and four circulating holes cut in the pipe 4" above the guide shoe with two cement baskets installed below these holes. After the casing was installed, mud was circulated for forty-five minutes. A wiper plug was installed at the top of the casing ahead of cement prepared by mixing 550 sacks of Colton Class G, 1:1 Perf-A-Lite, 40 percent silica flour, 2 percent gel, and an accelerator. The wiper plug was forced down the casing ahead of the cement thus keeping the cement separated from any mud and wiping the walls of the casing free of mud. After the Class G cement, a more rapid setting cement formed by mixing 44 sacks of Universal Atlas Lumnite cement (a calcium aluminum cement), 20 percent silica flour and a retarder was pumped down the casing. The cement was then displaced with 500 cubic feet of mud. After five hours and forty-five minutes the casing was placed in tension by applying a load of 180,- 000 pounds resulting in an elongation in the casing of 7". After tension was applied, it was found that cement was 70 feet down from thesurface and the cement level was brought to the surface by adding 75 sacks of Colton Class G cement. Although the Lumnite cement was allowed to set for 5% hours, it showed good compressive strength after four hours. The Perf-A-Lite cement had a fluid life of approximately eight hours, set in twelve hours and showed 360 psi. compressive strength in sixteen hours.

It can readily be seen to those skilled in the art that the present invention is not limited to the specific embodiments disclosed. Thus, any manner of cementing the casing may be used so long as the lower portion of the casing is anchored in the formation prior to setting of the cement in the upper portion of the casing-formation annulus. Furthermore, the casing may be anchored by methods other than cementing, e.g., by a pin connecting the casing with the formation wall. Furthermore, any desired method of applying stress to the casing during cementing of the cement may be used.

Having fully described the present invention, it is understood that it is not to be limited to the specific details set forth, but rather is of the full scope of the apended claims.

We claim:

1. A method of cementing a casing in a borehole to form a thermally stable, pre-stressed bond comprising introducing a relatively slow setting cement into the casing-borehole annulus, introducing a relatively rapid setting cement into the annulus to displace the relatively slow setting cement in the lower portion of the annulus and to force the relatively slow setting cement into the upper portion of the annulus whereby the relatively rapid setting cement sets and anchors the lower portion of the casing, applying stress in tension to the casing after the relatively rapid setting cement has set and prior to the setting of the relatively slow setting cement in the upper portion of the annulus, and maintaining the casing in a stressed condition until the relatively slow setting cement has set.

2. The method of claim 1 wherein the casing is stressed so that the elongation is at lea-st equal to the elongation which would occur freely in a casing at a temperature equal to the maximum temperature to which 6 References Cited UNITED STATES PATENTS 3,219,110 11/1965 Martin et al. 166-25 X 2,071,389 2/1937 Crowell 166-26 2,168,735 8/1939 Gilstrap 166-26 2,188,589 1/1940 Armentrout 166-47 2,206,389 7/ 1940 Cannon 166-28 X 2,236,987 4/1941 Bechtold 166-29 X 3,055,424 9/1962 Allen 166-21 3,277,963 10/1966 Flickinger 166-21 3,360,046 12/1967 Johnson et al 166-29 OTHER REFERENCES Shryock, Stanley H., et a1. thermal well completions can be improved. In World Oil, October, 1966, pp. 146- 149.

DAVID H. BROWN, Primary Examiner I. A. CALVERT, Assistant Examiner U.S. Cl. X.R. 

